1. Field of the Invention
The present invention relates generally to wavelength division multiplexing (WDM) using a plurality of optical signals having different wavelengths, and more particularly to an optical transmission device and an optical communication system applied to WDM.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying signal light has been put to practical use.
An optical amplifier known in the art comprises an optical amplifying medium to which signal light to be amplified is supplied and means for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) comprises an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 xcexcm band (0.97 xcexcm to 0.99 xcexcm) or a 1.48 xcexcm band (1.47 xcexcm to 1.49 xcexcm), a gain band including a wavelength of 1.55 xcexcm can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
Accordingly, by combining an optical amplifier and WDM, the span and capacity of an optical communication system can be increased.
In the case of combining an optical amplifier and WDM, there is a possibility that a transmission quality may be deteriorated by automatic output level control (ALC) performed in the optical amplifier. In general, ALC is control such that a total output level of an optical amplifier is maintained constant. Accordingly, when an optical signal in a certain one of WDM channels is cut off, for example, an optical output level in each of the other channels increases to cause a possibility that the transmission quality may be influenced by nonlinear effects (SPM: Self-Phase Modulation, XPM: Cross-Phase Modulation, FWM: Four-Wave Mixing, etc.) occurring in an optical fiber transmission line. It is known that the influence of nonlinear effects is remarkable particularly in the case of high-speed transmission at 10 Gb/s or higher.
It is therefore an object of the present invention to provide an optical transmission device and an optical communication system which can eliminate the possibility of deterioration in transmission quality in the case that the number of WDM channels is changed.
Other objects of the present invention will become apparent from the following description.
In accordance with an aspect of the present invention, there is provided an optical transmission device comprising an optical multiplexer for wavelength division multiplexing a plurality of optical signals to generate WDM signal light and outputting the WDM signal light to an optical transmission line; means for detecting a break of each of the plurality of optical signals according to the power of each optical signal; and compensating means for adding light having a predetermined wavelength to the WDM signal light when at least one of the plurality of optical signals is cut off.
With this configuration, when at least one of the WDM channels is cut off, the light having the predetermined wavelength is added to the WDM signal light. Accordingly, in the case of carrying out ALC (automatic output level control) for maintaining constant a total output level of an optical amplifier for amplifying the WDM signal light, for example, a change in optical output level per channel can be suppressed, thereby achieving one of the objects of the present invention.
In accordance with another aspect of the present invention, there is provided an optical transmission device comprising a plurality of optical senders for outputting a plurality of optical signals having different wavelengths; an optical multiplexer for wavelength division multiplexing the plurality of optical signals to generate WDM signal light and outputting said WDM signal light to an optical transmission line; and at least one light source for adding light having a predetermined wavelength to the WDM signal light.
In the case that ALC for maintaining a total output level constant is carried out in an optical amplifier, the range of variation in optical output level per channel is dependent on the number of channels. For example, when the number of channels decreases from 2 to 1, an optical output level of 3 dB is increased. In contrast therewith, when the number of channels decreases from 8 to 7, a change in optical output level is as small as 0.58 dB. Accordingly, by adding the light having the predetermined wavelength to the WDM signal light according to the present invention, the range of variation in optical output level in each of the remaining channels can be suppressed in the case that an optical signal in a certain one of the WDM channels is cut off.
In accordance with a further aspect of the present invention, there is provided an optical communication system. This system includes a terminal station apparatus for transmission and an optical fiber transmission line operatively connected to the terminal station apparatus. The terminal station apparatus includes the optical transmission device according to the present invention.
In this specification, the wording that an element and another element are operatively connected includes the case that these elements are directly connected, and also includes the case that these elements are so provided as to be related with each other to such an extent that an electrical signal or an optical signal can be mutually transferred between these elements.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.